Re: [PATCH v6 3/4] sched/fair: Introduce SIS_CORE

[Abel Wu]

On 10/21/22 12:34 PM, Chen Yu wrote:
> On 2022-10-21 at 12:30:56 +0800, Abel Wu wrote:
> > Hi Chen, thanks for your reviewing!
> >
> > On 10/21/22 12:03 PM, Chen Yu wrote:
> > > On 2022-10-19 at 20:28:58 +0800, Abel Wu wrote:
> > > [cut]
> > > > A major concern is the accuracy of the idle cpumask. A cpu present
> > > > in the mask might not be idle any more, which is called the false
> > > > positive cpu. Such cpus will negate lots of benefit this feature
> > > > brings. The strategy against the false positives will be introduced
> > > > in next patch.
> > > I was thinking that, if patch[3/4] needs [4/4] to fix the false positives,
> > > maybe SIS_CORE could be disabled by default in 3/4 but enabled
> > > in 4/4? So this might facilicate git bisect in case of any regression
> > > check?
> >
> > Agreed. Will fix in next version.
> >
> > > [cut]
> > > > + * To honor the rule of CORE granule update, set this cpu to the LLC idle
> > > > + * cpumask only if there is no cpu of this core showed up in the cpumask.
> > > > + */
> > > > +static void update_idle_cpu(int cpu)
> > > > +{
> > > > +	struct sched_domain_shared *sds;
> > > > +
> > > > +	if (!sched_feat(SIS_CORE))
> > > > +		return;
> > > > +
> > > > +	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> > > > +	if (sds) {
> > > > +		struct cpumask *icpus = to_cpumask(sds->icpus);
> > > > +
> > > > +		/*
> > > > +		 * This is racy against clearing in select_idle_cpu(),
> > > > +		 * and can lead to idle cpus miss the chance to be set to
> > > > +		 * the idle cpumask, thus the idle cpus are temporarily
> > > > +		 * out of reach in SIS domain scan. But it should be rare
> > > > +		 * and we still have ILB to kick them working.
> > > > +		 */
> > > > +		if (!cpumask_intersects(cpu_smt_mask(cpu), icpus))
> > > > +			cpumask_set_cpu(cpu, icpus);
> > > Maybe I miss something, here we only set one CPU in the icpus, but
> > > when we reach update_idle_cpu(), all SMT siblings of 'cpu' are idle,
> > > is this intended for 'CORE granule update'?
> >
> > The __update_idle_core() is called by all the cpus that need to go idle
> > to update has_idle_core if necessary, and update_idle_cpu() is called
> > before that check.
> I see.
>
> Since __update_idle_core() has checked all SMT siblings of 'cpu' if
> they are idle, can that information also be updated to icpus?

I think this will simply fallback to the original per-cpu proposal and
lose the opportunity to spread tasks to different cores.